slides from a talk about smart cards security on example of breaking once popular Cryptoflex cards.

1. On Smart Cards Security
Ilya O. Levin
D’Crypt Pte Ltd
Asiacrypt 2010

2. Common Beliefs
Obscure security tokens have security issues
An ISO-compliant smart card is more secure
than an obscure security token
Attacking smart cards is hard

3. Smart Cards
Physicalize cryptographic secrets by having
non-exportable objects (keys, PINs, etc.)

4. Schlumberger Cryptoflex

5. Cryptoflex Secrets
Elementary Files with Read AC set to
“Never Allowed”
0000 Cardholder Verification File (CHV1)
0001 Internal keys (DES, 3DES)
0011 External keys (AUT, DES, 3DES)
0012 RSA private keys (1012 – public)
0100 Cardholder Verification File (CHV2)

6. CHV File Format
Byte(s) Description Length
1 File activation byte, LSB 1
2-3 RFU 2
4-11 PIN value 8
12 Number of verification attempts allowed 1
13 Remaining verification attempt counter 1
14-21 Unblocking PIN value 8
22 Number of unblocking attempts allowed (10) 1
23 Remaining unblocking attempt counter 1

7. Internal Key File Slot Format
Byte(s) Description Length
1 RFU (!=0) 1
2 Key length or 1
00 = last slot
01 = empty slot
3 Algorithm ID 1
00 = Single-length DES, 56-bit
02 = Double-length 3DES, 112-bit
4-11 or 4-19 Key value 8/16
12 or 20 RFU (!=0) 1

8. External Key File Slot Format
Byte(s) Description Length
1 RFU 1
2 Key length or 1
00 = last slot
01 = empty slot
3 Algorithm ID 1
00 = Single-length DES, 56-bit
02 = Double-length 3DES, 112-bit
4-11 or 4-19 Key value 8/16
12 or 20 Number of verification attempts allowed 1
13 or 21 Remaining verification attempts counter 1
14 or 22 Next key data, starting from a key length

9. RSA 1024-bit Private Key File Format
Byte(s) Description Length
1 Key block length, MSB = 01 1
2 Key block length, LSB = 43h (323 bytes) 1
3 Key number 1
4-67 Public modulus secret prime factor P 64
68-131 Public modulus secret prime factor Q 64
132-195 Inverse of the factor P (a = Q-1 mod P) 64
196-259 Private subexponent (c = Ks mod (P-1)) 64
260-323 Private subexponent (f = Ks mod (Q-1)) 64
… … …
EOF 00 00 00 3

10. Cryptoflex Authentication
CHV1 User PIN
CHV2 Admin PIN (optional)
AUT1 Transport Key (2nd slot of 3f00/0011)
Still cannot read EFs with Never Allowed AC

11. Cryptoflex Authentication
We will skip the details of all related issues here
Assume CHV and AUT1 are known
Ask me offline if interested

12. Cryptoflex Secrets. The Fun Part
The non-exportable secrets are stored in the
elementary files
We can bypass standard commands and modify content
directly with Update Binary/Update Binary Enciphered

13. Cryptoflex Secrets. The Fun Part
The non-exportable secrets are stored in the
elementary files
We can bypass standard commands and modify content
directly with Update Binary/Update Binary Enciphered
Elementary files are transparent, not linear
We can modify anything in chunks of any size or one
byte at a time at any offset

14. Cryptoflex Secrets. The Fun Part
How to extract the first 112-bit 3DES encryption
key from the Internal Key File?
1. Establish the card context and satisfy AC
2. Let E = DES_Block_Init(dummy)
3. for i in [4 … 19]
4. for b in [0 … 255]
5. Update the ith byte in EF 0001 with b
6. if ( DES_Block_Init(dummy) ≡ E ) print b

15. Cryptoflex Secrets. The Fun Part
How to extract the first RSA-1024 private key?
Same as a 3DES key.
Use RSA Signature (Internal Auth) command instead of
DES Block Init.
Update bytes 4-67 in EF 0012 to recover the secret factor P
and bytes 68-131 to recover the secret factor Q

16. Cryptoflex Secrets. The Fun Part
The PoC code recovers a 3DES key in ~5 min
and a private RSA-1024 key in ~20 min

17. Cryptoflex Anamnesis
It is possible to recover cryptographic keys out
of “non-exportable” objects
Read access condition restrictions on key files
are irrelevant
“We do not consider this to be a security issue”

18. To conclude
Smart cards are not that perfect in real life as
we may believe
Poking smart cards is fun, no shiny hardware
required
We may need more public research in this area

19. Thank you